Adding All Selection To A Combo Box On Search Form

I have a search form with unbound combo boxes that uses the combo box values as criteria for a query. The Row Source of cboCategory is based on the query:

SELECT Categories.ID, Categories.Category FROM Categories ORDER BY Categories.Category;

When I start the form and I don't select a value for the cboCategory combo box, the query just returns all the values. This is great. The problem is that if I make a selection in the combo box, I have no way of reverting it back to that initial value that would make the query return all categories - if I erase the combo box the query returns nothing. So I have to close the form and reopen it.

I tried adding an "All" value to the combobox like so:

SELECT Categories.ID, Categories.Category FROM Categories UNION Select Null as AllChoice , "(All)" as Bogus From Categories
ORDER BY Categories.Category;

but it does not seem to work. It just returns nothing instead of all the categories. How can I add a value of "All" to the combo box that will return all of the categories?